1. Field of the Invention
The present invention relates to an electric wire used in wiring in equipment such as electronic equipment or the like and, more particularly, to an electric wire attached to a terminal of an insulation displacement connector.
2. Related Background Art
An electric wire for an insulation displacement connector generally comprises a stranded conductor obtained by twisting a plurality of conductor wires and a covering layer covering the stranded conductor. The end portion of the electric wire is inserted under pressure between two contact portions of a terminal of an insulation displacement connector having a plurality of terminals and two contact portions of the terminal on the sides of the electric wire penetrate the covering layer and contact the stranded conductor and are electrically connected to the stranded conductor. The insulation displacement connector has a wedged portion called strain relieves so as not to allow removal of the electric wire once accommodated (see reference numeral 6 in FIG. 2).
Polyvinyl chloride (referred to as xe2x80x9cPVCxe2x80x9d hereinafter) or the like was conventionally used as the material constituting the covering layer of the above-described electric wire for insulation displacement connector because PVC is inexpensive and excellent in workability. However, PVC produces a toxic halogen gas during combustion and may generate very toxic dioxin during incineration for disposal. Also, the U.S. UL (Underwire Laboratories Inc.) standard requires that the electric wire should have a certain degree of fire retardancy. Therefore, a material which does not produce toxic gases such as a halogen gas or the like and has a certain degree of fire retardancy is required as the material constituting the covering layer of the electric wire.
Under these circumstances, for example, as described in Japanese Patent No. 2525982, a resin composition including a fire retardant such as aluminum hydroxide or magnesium hydroxide or the like in a polyolefin resin is irradiated with ionizing radiation, and the resultant material is used as the material for the covering layer.
The present inventors have found that use of the material disclosed in the prior-art reference as the covering layer of an electric wire for an insulation displacement connector suffers the following problem.
Namely, according to the findings of the present inventors, as shown in FIG. 1, when an electric wire 3 is inserted under pressure between two contact portions 2 of a terminal of an insulation displacement connector 1 to bring a conductor 4 in electric wire 3 into contact with two contact portions 2 of the terminal, a covering layer 5 tears to expose conductor 4 in electric wire 3 and this may cause the short-circuit in the connector. As shown in FIG. 2, covering layer 5 deforms and electric wire 3 cannot be properly mounted in connector 1. Electric wire 3 once mounted may be removed.
It is an object of the present invention to provide an electric wire capable of sufficiently preventing removal of the electric wire from an insulation displacement connector and sufficiently preventing exposure of the conductor.
The present inventors have made extensive studies to solve the above problems. Namely, as to whether in electric wires having covering layers made of various materials, conductors are exposed (referred to as xe2x80x9ccore exposurexe2x80x9d hereinafter), or the covering layers deform (referred to as xe2x80x9ccovering layer deformationxe2x80x9d hereinafter) when electric wires for insulation displacement connectors are inserted under pressure between two contact portions of a terminal of insulation displacement connectors, the present inventors have conducted investigations by calculating the frequencies of xe2x80x9ccore exposurexe2x80x9d and xe2x80x9ccovering layer deformationxe2x80x9d occurring in the electric wires for evaluation. The relative relationship between this result and the 100% tensile modulus and elongation of the covering material was examined. The result is shown in FIG. 3. In FIG. 3, when the frequency of xe2x80x9ccore exposurexe2x80x9d or xe2x80x9ccovering layer deformationxe2x80x9d is {fraction (1/20)} or less, the result is determined as xe2x80x9cgoodxe2x80x9d and indicated by xe2x80x9c◯xe2x80x9d. When the frequency is higher than {fraction (1/20)}, the result is determined as xe2x80x9cpoorxe2x80x9d and indicated by xe2x80x9c▴xe2x80x9d for xe2x80x9ccovering layer deformationxe2x80x9d, and xe2x80x9cxc3x97xe2x80x9d for xe2x80x9ccore exposurexe2x80x9d. Also, in FIG. 3, G100 represents Y=7.8, while G200 represents E1=270xe2x88x928.5xc3x9710xe2x88x926xc3x97Y. A stranded conductor whose conductor size is AWG26 (obtained by twisting seven tinned wires each having a diameter of 0.16 mm) was used as a conductor of an electric wire for insulation displacement connectors. Covering layer is obtained by covering a resin composition having a predetermined composition and extruded by an 50-mm diameter extruder on the stranded conductor to have a conductor outer diameter of 0.98 mm, and then irradiating an electron beam at predetermined doses to the resin composition. Also, a JST DA connector having 5 terminals arrayed at the pitch of 2 mm (manufactured by JST Mfg Co., Ltd) was used as an insulation displacement connector.
The present inventors have found that the above problems in prior art can be solved by using a covering material such that its 100% tensile modulus and elongation satisfy predetermined conditions, thus accomplishing the present invention.
Namely, the electric wire of the present invention comprises a conductor and a covering layer covering the conductor, the conductor being brought into contact with two contact portions of a terminal by inserting the electric wire under pressure between two contact portions of the terminal of an insulation displacement connector, wherein the covering layer is made of a covering material obtained by irradiating with ionizing radiation a resin composition containing an ethylene copolymer and a metal hydroxide surface-treated with a silane compound represented by: 
(where R represents an alkyl group having an acrylic, methacrylic, or allyl group, a saturated alkyl group, a vinyl group, an epoxy group, an amino group, or a mercapto group; X1, X2, and X3 represent alkoxy or alkyl groups, respectively; and at least one of X1, X2, and X3 represents an alkoxy group), a 100% tensile modulus of the covering material is not less than 7.8 MPa, and the 100% tensile modulus and an elongation of the covering material satisfies the following relationship:
E1 greater than 270xe2x88x928.5xc3x9710xe2x88x926xc3x97Y
(where E1 is the elongation and Y is the 100% tensile modulus).
According to the present invention, in inserting under pressure the electric wire between two contact portions of the terminal of the insulation displacement connector, the electric wire can be reliably mounted in the insulation displacement connector without deforming the covering layer, and the removal of the electric wire once mounted can be prevented. Also, damage to the covering layer and exposure of the conductor can be sufficiently prevented.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.